Spectra• All electromagnetic radiation travels in waves at

the same velocity, commonly known as the speed of light.

• In a vacuum this value is known to be 2.998 x 108 meters per second.

• Electromagnetic radiation differs in wavelength () and frequency ().

• The product of the wavelength and frequency is equal to the speed of light.

C.. • The energy of a wave is proportional to its

frequency: E = h,

where h = Planck's constant or 6.626 x 10-34 J s.

Spectra and Organic Molecules

• When light strikes an organic molecule, some of its energy is absorbed.

• Molecules are affected in various ways depending on the energy of the light.

• If the light has a wavelength in the range from about 400 to 700 nm, it falls in the visible region.

• When light of a particular wavelength is absorbed, the compound is seen as having a color. This is especially true of organic dyes,

Absorption Spectra

• If electromagnetic radiation with wavelengths outside of the visible region is absorbed, the results cannot be observed directly. •H owever the absorption can be detected using a spectrophotometer.

Absorption Spectra and Molecular Characteristics

•Many of the functional groups that are found in organic molecules absorb infrared at characteristic frequencies.

•This energy that is absorbed is converted to various kinds of molecular vibrations including the rhythmic stretching of chemical bonds,

•Each type of chemical bond has a characteristic absorbance in the infrared region.

Absorption depends on the Molecular Environment

•The exact frequency of absorption depends on the specific environment of the bond.

•Some kinds of bonds appear in more than one in of compound.

Absorption peaks depends on functional groups

•For example, the absorption frequencies for a hydroxyl group, -OH, appear in approximately the same location whether it was in an alcohol or a carboxylic acid, but obviously it would not appear in a alkanone or ester. Both acids and alkanones show a strong absorption frequency for the carbonyl group C=O.

Typical Infrared Absorption frequencies

Functional group

Class of compound

Frequency of absorption in cm-1

Intensity of the absorption

OH Alcohols, phenols 3500-4650 Variable, sharp

OH Carboxylic Acids 2500-3000 Variable, broad

OH Hydrogen bonded alcohols and phenols

3200-3400 Strong, broad

C-O Alcohols, ethers, esters, and carboxylic acids

1080-1300 Strong

C=O Alkanones, alkanals, esters, and carboxylic acids

1690-1750 Strong

C-H Alkanes 2850-2950 Strong

N-H amines 3300-3500 Medium       

Typical Infrared Spectra -- Cyclohexanol

Typical Infrared Spectra -- 1 Octanol

Typical Infrared Spectra -- 2 bromobutanoic acid

Typical Infrared Spectra -- 2 hydroxybenzaldehyde

Typical Infrared Spectra -- 2 aminomethylbenzene

1 Propanol

1 propanol

2 Butanol

2 butanol

Ethyl Methanoate

Benzaldehyde

3 Pentanone

Acetophenone